ralloc: Fix ralloc_adopt() to the old context's last child's parent.

ralloc: Fix ralloc_adopt() to the old context's last child's parent.

I was cleverly using one iteration to obtain a pointer to the last item
in ralloc's singly list child list, while also setting parents.

Unfortunately, I forgot to set the parent on that last item.

Cc: "11.1 11.0 10.6" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>